For 25 years, through four presidential administrations, U.S. schools could rely on one small truth: Math scores on the National Assessment of Educational Progress, or NAEP test, always went up.

Not this year.

The latest results of the biennial tests, given to thousands of students and nicknamed "The Nation's Report Card," show a first-ever drop in math scores for the randomly selected students in both fourth- and eighth-grade students who took them earlier this year.

U.S. Education Secretary Arne Duncan acknowledged that the news "isn't great," but said the dip "doesn't come as a big surprise." Peggy Carr, the federal official who oversees the tests, cautioned against reading too much into the development, saying, "One downturn does not a trend make." Carr said she would withhold judgment until 2017.

The new scores, released Wednesday by the National Center for Education Statistics, show the average fourth- and eighth-grade math scores declined two and three points, respectively, between 2013 and 2015. The average eighth-grade reading score dropped three points. Fourth-grade reading scores were essentially unchanged.

President Obama listens as Education Secretary Arne Duncan speaks in the State Dining Room of the White House on Oct. 2, 2015.(Photo: Andrew Harnik, AP)

Since 1990, scores in both math and reading have moved steadily, if slowly, higher. Until this year, math scores had never dropped in either grade. NAEP scale scores range from 0 to 500.

Carr ruled out "test fatigue" on the part of students, saying researchers who looked into that found no evidence that students in 2015 were any less engaged in the test than in past years.

But even Carr, a developmental psychologist, said the drops surprised her.

"This isn't a pattern that we saw coming," she said. "In that sense, it was an unexpected downturn. But we'll see. I think the bigger point is that we'll see if this is going to be a trend that will continue."

The Obama administration last week moved to limit the time students spend taking and preparing for standardized tests, saying testing is "consuming too much instructional time and creating undue stress for educators and students." In its plan to reduce how much testing takes place in school, the administration took at least part of the blame for "unnecessary" testing without a clear purpose.

Carr said NAEP tests stand apart from the typical standardized tests most kids endure. NAEP tests are taken by a small fraction of students, for one thing, and take only about an hour to complete. They can't be prepped for or gamed and the scores, reported anonymously, have no stakes attached. They're not part of students' school records. Students don't even know they've been chosen to take them until shortly before they're called out of class to sit for the exams.

Observers were quick to offer theories on the drop — everything from the recent recession to the rise of the new, more challenging Common Core curriculum.

Michael Petrilli of the Thomas B. Fordham Institute, a Washington, D.C., education think tank, predicted that states with big declines in median income in recent years would produce lower NAEP scores. "It makes sense," he said. "When families are hurting financially, it's harder for students to focus on learning."

Chris Minnich, executive director of the Council of Chief State School Officers, suggested that curricula have changed for many schools nationwide. As states raise standards, he said, "it will take time for students and teachers to adjust."

He also suggested that there may be differences between what schools now teach and what NAEP tests. "NAEP is one measure and it must be considered in the context of other measures of student achievement, such as state test scores, graduation rates, remediation rates, college entrance exams, and others," he said.

Among the data were a few bright spots. Several big cities showed promising results. Average scores for large cities held steady over the past two years and rose in three urban districts: Miami-Dade County, Chicago and Washington, D.C..

In D.C., average fourth-graders scored higher in both math and reading. D.C. Public Schools (DCPS) Chancellor Kaya Henderson called the development "astounding" and noted that since 2011, city fourth-graders have improved 13 points in reading and 10 points in math, even as fourth-graders nationally have been flat in math and up only two points in reading.

Henderson credited a new Common Core-aligned curriculum, among other measures, for the improvements.

"When you look at the national results, DCPS is outpacing the nation," she said. City schools have even outpaced many independently run charter schools. "We're really proud," she said. "We're demonstrating that the trope about districts being unfixable is not true."

Like Carr, Duncan cautioned about making too much of the math downturn, saying efforts to raise academic standards take years. "Big change never happens overnight," he said.

Duncan added that scholars would likely be puzzling over the new results for years to figure out what's happening.

"Anyone who claims they have all this figured out has a personal agenda rather than an educational one," he said.